1. Field of the Invention
The present invention relates to a wavelength conversion element and a light emitting device.
2. Description of Related Art
A light emitting device obtaining a white light by making a phosphor emit a light by means of a light from a light emitting diode (LED) element as an excitation light was hitherto developed in the uses of illumination and the like.
As such a light emitting device, for example, the following light emitting devices are known: one using a phosphor emitting a yellow light generated by being excited by a blue light emitted from an LED element to make a white light by mixing the color of each light with each other; one using a phosphor emitting a blue light, a green light, and a red light generated by being excited by an ultraviolet light emitted from an LED element to make a white light by mixing the three color lights emitted from the phosphor with one another; and the like.
Although a light emitting device made by directly sealing an LED chip with a hardening resin in which a phosphor is dispersed was developed as a configuration of such a light emitting device, the uses of the light emitting device have expanded to a region in which high luminance is required like a headlight of an automobile or the like, and now the heightening of the output power of white LED's has advanced to cause the heat generation of the LED chips. Consequently, if a phosphor is directly provided on an LED element in the form of being dispersed in a sealing medium as described above, the phosphor sometimes thermally deteriorates owing to the heat generated by the LED element.
A technique of preventing the deterioration of a sealing agent by dispersing a phosphor not into a resin but into a ceramic layer to seal an LED was proposed in order to settle such a problem (see, for example, Japanese Patent Application Laid-Open Publication No. 2000-349347). The ceramic layer is formed by mixing a phosphor into a precursor solution of the ceramic layer to bake the mixture.
However, the specific gravity of a phosphor is generally very large, e.g. about 4, and it is impossible to prevent the precipitation of the phosphor in the precursor solution of the ceramic layer. It is therefore impossible to uniformly disperse the phosphor in the ceramic layer. In the ceramic layer, the generation of a crack (the so-called breakage) is regarded as a problem, and it was also proposed to use ceramic powder and mix a phosphor with the ceramic powder for the purpose of preventing the crack. The countermeasure is however insufficient yet.
Furthermore, the technique disclosed in Japanese Patent Publication No. 4308620 (see paragraphs 0057-0059, FIG. 3, and the like) provides a wavelength conversion part (such as a luminescence conversion layer 4) at a position distant from a semiconductor device separately. In this form, because the wavelength conversion part is away from the semiconductor device, it is considered that the phosphor can be prevented from being thermally deteriorated by the heat generated by the semiconductor device.
However, even in such a form, because the specific gravity of the phosphor is large as described above, the phosphor precipitates when the phosphor is simply mixed with a precursor of the ceramic layer, and a uniform ceramic layer cannot be made.
Accordingly, the present inventor examined these techniques, and found that it was possible to prevent the precipitation of a phosphor to uniformly disperse the phosphor into a ceramic layer, by mixing fine particles of an oxide with the precursor solution of a ceramic layer (a liquid obtained by dissolving a ceramic precursor into a solvent) and by using alkoxysilane or a compound having a plurality of siloxane structures among the precursors.
However, although it was possible to obtain a good dispersion state of a phosphor by mixing fine particles of an oxide with a solution of alkoxysilane or a compound having a plurality of siloxane structures, it was found that a problem of the occurrence of a crack in the ceramic layer at the time of being baked and a problem of the exfoliation of the ceramic layer from a substrate supporting the ceramic layer were produced if the fine particles of the oxide were simply mixed. The smaller the fine particles of the oxide are, the larger the specific surface areas per unit weight are. Consequently, if the fine particles of the oxide are too much mixed, it becomes difficult to cover the surfaces of the fine particles with the ceramic precursor, and it is necessary to make sure the proper quantity of the mixing quantity of the fine particles of the oxide.